Amorphous Si0.9C0.1H and Si0.7C0.3:H alloys were deposited by magnetron sputtering onto a non-heated substrate, using benzene vapour as a source of carbon atoms. The specimens were exposed to sequential isochronal thermal annealing, up to 1050 degrees C, in a vacuum chamber, followed by IR and Raman spectroscopy measurements. The influence of the thermal treatment on the structural ordering was monitored by the evolution of the intensity and shape of the characteristic bands corresponding to the Si-H, SI-C, C-C and Si-Si bends. At low temperatures, up to 400 degrees C, the most pronounced features are accompanied by hydrogen evolution, the appearance of new Si-C bonds and an increase of disorder in the material. Above 600 degrees C, the structural ordering begins and between 800 and 1000 degrees C the crystalline Si-C and C-C phases appear. The results are discussed using a model of continuous random network of silicon and carbon atoms interrupted by voids.